In order to meet the emission level requirements for industrial low emission gas turbine engines, staged combustion is required in order to minimise the quantity of the oxides of nitrogen (NOx) produced. Currently the emission level requirement is for less than 25 volumetric parts per million of NOx for an industrial gas turbine exhaust. The fundamental way to reduce emissions of nitrogen oxides is to reduce the combustion reaction temperature, and this requires premixing of the fuel and all the combustion air before combustion takes place. The oxides of nitrogen (NOx) are commonly reduced by a method which uses two stages of fuel injection. Our UK patent no. 1489339 discloses two stages of fuel injection. Our International patent application no WO92/07221 discloses two and three stages of fuel injection. In staged combustion, all the stages of combustion seek to provide lean combustion and hence the low combustion temperatures required to minimise NOx. The term lean combustion means combustion of fuel in air where the fuel to air ratio is low, i.e. less than the stoichiometric ratio. In order to achieve the required low emissions of NOx and CO it is essential to mix the fuel and air uniformly so that it has less than a 3.0% variation from the mean concentration before the combustion takes place.
The industrial gas turbine engine disclosed in our International patent application no WO92/07221 uses a plurality of tubular combustion chambers, whose longitudinal axes are arranged in generally radial directions. The inlets of the tubular combustion chambers are at their radially outer ends, and transition ducts connect the outlets of the tubular combustion chambers with a row of nozzle guide vanes to discharge the hot gases axially into the turbine sections of the gas turbine engine. Each of the tubular combustion chambers has an annular secondary fuel and air mixing duct which surrounds the primary combustion zone. Also each of the tubular combustion chambers of the three stage variant has an annular tertiary fuel and air mixing duct which surrounds the secondary combustion zone.
A set of primary fuel injectors is provided to supply fuel into the primary combustion zone, a set of secondary fuel injectors is provided to supply fuel into the upstream end of the secondary fuel and air mixing duct and a set of tertiary fuel injectors is provided to supply fuel into the upstream end of the tertiary fuel and air mixing duct. Each of three sets of fuel injectors requires its own fuel manifold. This requirement for three sets of fuel injectors and fuel manifolds makes the arrangement very complicated.